Improvement of nonlinearity and extension of wavelength region using tandem (PV+PC) type HgCdTe detector (dual-MCT) in FTIR spectrometer

Journal of Electronic Materials, Jun 1999 by Abe, O, Kawasaki, K, Wakaki, M

A tandem (PV PC ) type HgCdTe detector (Dual-MCT) was fabricated to improve the nonlinear detector response and extend the wavelength region in Fourier transform infrared spectrometer. The backside-illuminated narrow band PV-MCT detector was arranged at the upper part of the hybrid device. A grid or a circular pattem electrode was formed on the active area to transmit the longer wavelength light than cutoff. The appropriate reverse bias was applied to the detector to obtain better linearity. The PC-MCT detector with a longer cutoff wavelength of middle band or wide band was arranged at the down part of the device to receive the transmitted light through the PV-MCT. The interferogram signals with better linearity from both detectors were combined after the phase matched preamplifier and Fourier transformed to a spectrum. The calibration curves evaluated with the absorption peaks at wavenumbers of 2025 and 972 cm^sup -1^ of n-- hexane showed that the Dual-MCT had excellent linearity and wider wavelength coverage.

Key words: Dual-MCT, Fourier transform infrared (FTIR), HgCdTe detector, interferogram, nonlinear detector response, spectrometer

INTRODUCTION

In the field of a infrared spectroscopy, recent development of Fourier transform infrared (FTIR) spectrometers using a two beam interferometer made possible simultaneous photometry, and contributed to achieve large improvement of SIN and shortening of measuring time compared with conventional dispersion type spectrometers. These improvements owe to two nominal merits of a FTIR spectrometer. These are 1) multiplex advantage of Fellgett, and 2) throughput advantage of Jacquinot. These advantages imply larger number of photons impinge into the detector compared with dispersion type spectrometry, which arise no problem in linear response thermal type detectors like a TGS detector. However, a degradation of spectral response often becomes a problem for a photoconductive high sensitive quantum type detector like typically photoconductive HgCdTe(MCT) detector.1,2

A photoconductive (PC) type HgCdTe (MCT) detector is the typical high sensitive IR detector well installed for versatile FTIR spectrometers according to high cost performance and tunability of wavelength coverage in a middle IR region. However, nonlinearity of a output signal vs incident light intensity is eminent and a dynamic range is not so wide.3 The amplitude of the detected signal as a interferogram is modulated due to the nonlinear response and mainly the center burst of the interferogram is compressed, then processed to Fourier transform.4 In this case, the measured spectrum is the convolution between a real spectra and a Fourier transformed modulation function. As a result, spectral profile becomes distorted and false-peak known as nonphysical energy is appeared in the longer wavelength region where the detector has no sensibility.5 Several methods for the correction of nonlinearity have been carried out using a software and/or a pre-amplifier like a linearizer6 and constant voltage driven one,7 which did not offer complete correction.

The problem of nonlinearity of the detector is improved largely by using a photovoltaic (PV) type MCT detector which has a very short time constant. A PV type detector has a feature to show better linear photo response by applying a reverse bias voltage, but has not been installed to versatile FTIR spectrometers due to the cost and the rather narrow wavelength range. We developed the dual-MCT which integrated a PV-type and a PC-type MCT detectors. This new improvement is based on the detector itself. This new type of detector has a feature to have better linearlity than conventional PC type MCT and be easy to broaden the wavelength region, and possible to install to any type of FTIR spectrometers of any maker.

DEVICE CONCEPT

Linearity of the optical response can be improved largely by using a PV type MCT instead of a PC type MCT, as mentioned before. However, the installation of a PV type MCT to a versatile FTIR spectrometer is of little merit due to its high cost and a narrow spectral region. The wide band PV type MCT extending to about 19 (mu)m was reported by Pultz et al. ,8 and R&D targeting the wide band response is steadily carried out. But needs are restricted to the expensive 2D arrays and the possibility to join the cost competing FTIR spectrometer market is very small. For versatile FTIR spectrometers, it is reasonable in cost to use a PC type MCT and compensate nonlinearity by using a software. However, such an indirect method does not give a complete solution. The complete solution may be offered by good linearity of the detector itself. We developed a versatile Dual-MCT in point of such a view.

We considered to use a PV type MCT with cut-off wavelength of about 10 (mu)m, which was easily obtainable. A PV type MCT is possible to fabricate, because today the wafer with a high quality HgCdTe heterostructure is available commercially. The basic concept of our development is as follows. A PV type MCT covers the wavelength range to around 10 (mu)m. A MCT crystal transmits a longer wavelength ray than a cutoff. The transmitted light is received by a PC type MCT to extend a sensitive wavelength region. The output signal from the PV type MCT is a linear signal. On the other hand, the light intensity incident into the PC type MCT transmitted through the PV type MCT crystal which act as a low pass filter is very small. Then, the PC type MCT operates at a linear response region and gives a highly linear output signal. Both signals are combined at the preamplifier. The concrete arrangement is shown in Fig. la and described below.